This invention relates generally to electrostatic precipitators for air pollution control and, more specifically, concerns the control of the rapping process used to clean the internal collection plates and discharge electrodes of electrostatic precipitators.
Continuous emphasis on environmental quality has resulted in increasingly strenuous regulatory controls on industrial emissions. One technique which has proven highly effective in controlling air pollution has been the removal of undesirable particulate matter from a gas stream by electrostatic precipitation. An electrostatic precipitator is an air pollution control device designed to electrically charge and collect particulates generated from industrial processes such as those occurring in cement plants, pulp and paper mills and utilities. Particulate laden gas flows through the precipitator where the particulate is negatively charged. These negatively charged particles are attracted to, and collected by, positively charged metal plates. The cleaned process gas may then be further processed or safely discharged to the atmosphere.
During continuous operation of an electrostatic precipitator, the collector plates, electrodes and other precipitator internal components must be periodically cleaned to remove the dust build-up which accumulates on these surfaces. The cleaning mechanism typically consists of a mechanical rapper. An electronic rapper controller determines the sequence, intensity, and duration of rapping. Once the particulate is dislodged from the plates, it falls into collection hoppers at the bottom of the precipitator.
Rappers are electromechanical devices that are used to mechanically dislodge collected particulate/materials within an electrostatic precipitator, electronic filter or dust collector (hereafter referred to as ESP) by applying direct current (DC) energization to the rapper. In general, a rapper consists of a hammer that mechanically strikes an anvil. The anvil is mechanically connected to the internal components of the ESP, such as the discharge electrodes, collecting plates, gas distribution devices or any other component cleaned by the rapper. Striking the rapper shaft or anvil with the hammer transmits mechanical forces to these components to dislodge collected materials. Several rapper variations exist which may be employed in the cleaning process.
One rapper variation consists of a cylindrical hammer or plunger and solenoid coil. The hammer rests on the rapper shaft or anvil. When the solenoid coil is energized with a DC voltage the resulting electromagnetic force overcomes the force of gravity and lifts the hammer vertically to a height that is determined by the amplitude and length of time of the energization. When said energization is terminated, the electromagnetic field is removed and the hammer drops due to gravitational forces and strikes the anvil. The hammer then rests on the anvil until the next energization.
Another rapper variation places a spring behind the cylindrical hammer. When the solenoid coil is energized with a DC voltage the resulting electromagnetic force will overcome the force of gravity and lift the hammer vertically compressing the spring against the rapper assembly. The height and spring compression are determined by the amplitude and length of time of the energization. When the energization is terminated the hammer strikes the anvil with a force that is comprised of gravitational force plus the spring expansion.
Another rapper variation places a spring behind the cylindrical hammer. This spring is connected to the hammer and holds it above the anvil. When the solenoid coil is energized with a DC voltage the resulting electromagnetic force will overcome the force of the spring and accelerate the hammer downward to strike the anvil. When the energization is terminated, the hammer is returned to position by the spring.
The energization of the rapper solenoid coil induces a flow of electromagnetic flux around this coil and which also flows through the cylindrical hammer and other rapper components. In addition, there are stray undesirable flows of electromagnetic flux some of which pass through the anvil assembly. The amount of undesirable electromagnetic interaction with the anvil and other components is dependent upon the type and construction of the rapper.
Since the energization is DC, and therefore unipolar (i.e., one direction), the components are exposed to repeated electromagnetic energization with the same orientation of North and South poles. To illustrate, the electromagnetic flux flow radiates from the coil in the same direction every time it is energized. The flow direction moves outward from the coil and around to the bottom of the anvil through the anvil's top, then upward through the bottom of the hammer and out through its top. Because the hammer and anvil are separated, each will have a North and South pole. As this unipolar energization is repeated, the North and South poles of both the hammer and anvil become stronger until they retain their magnetic orientation. As stated by Lenz's Law, this induced magnetic effect is proportional to the amount of energy used to create it and will, therefore, oppose it with like force. On successive energizations, therefore, this residual magnetization will oppose lifting the hammer to its desired level. This is particularly true when the hammer rests on the anvil.
The result of this residual magnetization is that the anvil and hammer are of different poles and are therefore attracted to each other and may not lift at all when the energization is at a low level. The length of time and amplitude of the energization are varied to adjust the intensity of the rapper.
In practice, numerous operational problems associated with the cleaning process may be experienced. Excessive rapping results in the particulate billowing from the plate into the gas stream where it is re-entrained in gas flow and must be recaptured. Otherwise, the re-entrained dust will be discharged from the exhaust stack, resulting in unacceptable emissions into the atmosphere. Insufficient rapping prevents the particulate from falling from the surfaces to be cleaned. In either case, collection efficiency of the precipitator is reduced which reduces the gas volumes that can be treated by the precipitator. In most industrial applications there is a direct correlation between precipitator capacity and production capacity. Therefore, there are significant monetary benefits to be derived from maximizing rapper efficiency. Also, grossly inefficient precipitators which allow an excessive amount of particulate emissions into the atmosphere can prompt the Environmental Protection Agency to shut a particular process down indefinitely.
In the prior art, rapper control has been limited to manually controlling and adjusting the current level to an entire group of rappers, rather than individual rapper control. However, rappers in different locations within the group may operate more efficiently with different current levels. Since the number of rapper groups, as well as the number of rappers within each group, may vary and prior art rapper control only allows for intensity adjustment of an entire group, a compromise in control standards therefore prevails. The result is often rapper inefficiencies that reduce precipitator and production capacity as well as increase emission levels.
Similarly, open and short trip values must be set for the rappers as a unit. Since rappers at different locations may have different current protection requirements, the prior art represents yet another compromise. To protect the rappers as a unit the least sensitive rapper must de-energize when a circuit condition occurs that is threatening to the most sensitive rapper. This is inefficient since some rappers will at time be de-energized unnecessarily even though their particular operating parameters are not exceeded.
With respect to circuit protection, the prior art uses fuse or relay technology to detect and isolate fault conditions. This technology is slow in that the devices require up to several full cycles before electrical protection can be assured. Within several full cycles of a fault condition significant damage can occur to rapper circuitry. Some commercial rapper control systems purport to incorporate solid state fault detection, but the trip level is set high because all rappers are required to have the same trip level. The trip level cannot be individually adjusted to a specific single rapper within these systems and a compromise in control standards results.
Another drawback of the prior art is that rapper control technology is an open looped system. The current level is set at a particular point in time, considering the present rapper conditions in the electrostatic precipitator. But, rapper conditions are not static. Numerous things can change rapping conditions which often affect current flow to the rappers. For instance, the precipitator may operate at elevated temperatures which change the ambient temperature of the rapper. Rapper slugs as they energize travel through a sleeve which often gets dirty and sticky. Numerous influences change the rappers characteristics but the prior art requires control just as if the conditions are constant. This again results in inefficiencies.
The prior art does not provide an easy or economical way to check the present operating conditions of rappers in large precipitators. Presently, technicians must personally walk near each precipitator while watching and listening to determine whether a specific rapper is operating. To determine the present current flow to a rapper, or to determine what current a particular style of rapper draws, a technician must personally measure each rapper input with a meter. In large precipitators (for instance, 250 rappers or more) it becomes cost prohibitive to personally check the efficiency of each rapper.
Similarly, the prior art is unable to provide trending information for specific rappers, which can be very important in troubleshooting, calculating overall operating efficiencies, as well as calculating the useful life expectancies of specific rappers.
In addition, the prior art does not provide a means by which the undesirable, residual magnetism created within rapper components can be eliminated.
A long felt need in the air pollution control industry remains for improvements in rapper control for electrostatic precipitators to alleviate the many operational and maintenance difficulties which have been encountered in the past. The primary goal of this invention is to fulfill this need.
The present invention provides an improved way to control power to a rapper within an electrostatic precipitator.
Since manually adjusting current to rappers as a unit is inherently inefficient, an important object of this invention is to provide a means for individually pre-setting electrical operating conditions for each rapper within a multiple rapper precipitator.
Another object of this invention is to provide a means for individually setting short and open trip conditions for each rapper within a multiple rapper precipitator. This will eliminate the compromise required in the prior art and increase rapper efficiency.
Still another object of this invention is to provide fault protection which assures detecting and isolating a fault condition within 1/2 cycle from the moment a fault occurs. Reducing fault trip response times from several full cycles to 1/2 cycle will greatly increase circuit protection and increase the useful life expectancy of the rappers and precipitator as a whole.
Another object of this invention is to provide a closed-loop control means for a rapper. Enabling the rapper current control to sense, measure and adjust the input current in the event the actual current is not substantially similar to the pre-set electrical input current will greatly aid rapper efficiency.
Yet another important object of this invention is to provide a source of rapper energization that will reverse polarity every time an individual rapper is energized. Further, the polarity of energization will be remembered so that it can be reversed on the occurrence of each energization.
It is also an object of this invention to de-magnetize those rappers that have become magnetized with prior art controls.
Also, it is a further object of this invention to more accurately control rapper lift and thereby rapper intensity by eliminating the detrimental and inconsistent effects of residual magnetism on predicting consistent rapper lift.
Another important object of this invention is to provide present operating conditions for each rapper within a precipitator and to store the rapper operating conditions. This will provide an economical way to check the actual operating conditions of each rapper as well as provide information for troubleshooting and trending.
Other and further objects of the invention, together with the features of novelty appurtenant thereto, will appear in the course of the following description.